(1) In vitro carcinogenesis of chimpanzee skin in preparation for a primate cancer vaccine trial has been initiated. (2) Studies are under way to identify essential genes coding for malignancy by murine leukemia viruses. (3) Studies on the maintenance and behavior analysis of human and other xenotransplants in athymic nude mice are continuing. (4) Pilot experiments are in progress on photochemical inactivation of tumor cells, for possible therapeutic application, and transformation of normal cells to provide possible sources of immunoprotective tumor antigens for vaccine use.